This invention relates to an optical scanning device for scanning an optical record carrier, such as an optical disk, comprising at least one information layer, the device comprising a radiation source for generating a radiation beam and a compound objective lens system located in an optical path between the radiation source and the record carrier for converging the radiation beam to a spot on an information layer.
There is a need for the production of optical record carriers of high capacity. Optical disks have been devised that have multiple information layers in a single disk (referred to as a xe2x80x9cmultilayerxe2x80x9d disk). It is also possible to increase the amount of information stored on a disk by decreasing the size of the data markings. In order to read such small data markings, optical scanning devices using a relatively short wavelength and a high numerical aperture (NA) objective lens system, with at least NA greater than 1 and preferably NA= greater than 1.6 (an xe2x80x9cultra-highxe2x80x9d numerical aperture), are desirable.
U.S. Pat. No. 5,125,750 proposes an optical scanning device using an objective lens system of two lenses with the first lens element being a single objective lens well corrected for aberrations and the second lens being a solid immersion lens (SIL). The use of the SIL increases the numerical aperture of the system whilst reducing, or at least not introducing, unwanted aberrations as the beam enters the optical disk medium. There are two proposed configurations of such a dual-lens objective, the xe2x80x9ccentre of curvaturexe2x80x9d case and the Huygens aplanatic case. These configurations are described briefly below. For a more detailed explanation see W T Welford, Aberrations of Optical Systems (Adam Hilger, Bristol, 1991), pp. 158-161.
In the xe2x80x9ccentre of curvaturexe2x80x9d case, a single objective lens element is combined with a spherical plano-convex SIL having a spherical radius r and an axial thickness d substantially equal, i.e. d=r. With the beam properly focused on the exit surface of the SIL, it enters the SIL medium without refraction. The total numerical aperture of this system, in which the objective lens element has a numerical aperture NAOBJ and the SIL has a refractive index nSIL, is
NA=nSILxc2x7NAOBJ
From this equation it can be seen that the total numerical aperture of the system is increased when the SIL is used, by a factor equal to the refractive index of the SIL. The maximum refractive index obtainable in a SIL is approximately 2.2 and the maximum numerical aperture of a single lens objective lens is approximately 0.65. Therefore, the maximum obtainable numerical aperture is approximately equal to 1.4.
In the Huygens aplanatic case a single lens objective is combined with a spherical plano-convex SIL in which the spherical radius r and axial thickness d are related by the refractive index of the SIL nSIL, namely d=r+r/nSIL. With the beam properly focused on the SIL the beam is refracted and additionally focused on entering the SIL medium. The total numerical aperture of this system, in which the objective lens element has a numerical aperture NAOBJ, is
NA=n2SILxc2x7NAOBJ
with the constraint NA less than nSIL.
This arrangement may therefore be used to provide an ultra-high NA objective lens system.
The inventors recognize that tolerances in variations in thickness and radius of the SIL are low and these variations must be below 0.5 xcexcm. The SIL therefore becomes a relatively difficult component to manufacture. Furthermore, because of these strict tolerances the objective lens system is not capable of reading different layers of a multilayer disk.
In accordance with one aspect of the invention there is provided an optical scanning device for scanning an optical record carrier including at least one information layer, the device comprising a radiation source for generating a radiation beam and a compound objective lens system, located in an optical path between the radiation source and the location of the record carrier, for converging the radiation beam to a spot on a said information layer, wherein the compound objective lens system comprises a first lens element, a second lens element located between the first lens element and the location of the record carrier, and a third lens element located between the second lens element and the location of the record carrier, wherein the first lens element is arranged to converge the beam to a first convergent state, the second lens element is arranged to converge the beam to a second convergent state, said second state being of greater convergence than said first state, and the third lens element is arranged to supply the beam to the spot on the information layer.